PROJECTSUMMARY The endogenous endocannabinoid (eCB) system includes cannabinoid receptor 1 (CB1R), CB2R, and two endogenous ligands (N-arachidonoylethanolamine;? AEA and 2-arachidonoylglycerol, 2AG). Animal findings demonstrate that the eCB system undergoes dynamic changes during adolescence and suggest a significant role in neurodevelopment, especially in CB1R-dense prefrontal, parietal, striatal and limbic brain regions. Research in adults have found that eCB system signaling modulates executive functioning, reward response, and stress and affective processing. Thus, there is strong preclinical evidence that the eCB system plays a substantial role in neuronal activity and neurodevelopment and disruption of or reduced eCB signaling during adolescencemayresultinabnormalneurocognitivedevelopment,riskforpsychopathology,andincreasedrisk for substance use, especially cannabis (CAN) use. Despite this evidence, no study to date has examined the role of the eCB system on neurocognition, mood, or early onset cannabis use and early cannabis subjective effects ex vivo in human youth due to the inherent challenges in employing invasive techniques [i.e., PET studies or collecting CSF]. However, there is now an available bioassay to measure circulating AEA and 2AG in serum collected from blood, that can be measured ex vivo in humans. Serum AEA and 2AG levels reflect both peripheral system synthesis and overflow from the brainand animal evidence has shown that circulating and brain eCB concentrations are robustly and significantly correlated. Further, adult studies have found significant associations between circulating serum eCBs and executive functioning, stress response, reward signaling, affective processing, mood and anxiety symptoms and regular CAN use. Notably, studies relating serumeCBlevelsandneurocognitive,psychopathology,andCANuseoutcomeshavenotyetbeenconducted in youth- despite convincing preclinical evidence that the impact may be greater during these critical neurodevelopmentalyears.Thus,thisR21projectwillbethefirsttoestablishthelinkbetweencirculatingeCB levelsandneurocognitive,psychopathology,andearlyCANuseoutcomesinasubsetof2000youthaged11- 14 who are already enrolled in the longitudinal Adolescent Brain Cognitive Development (ABCD) Study (at 8 geographically-diverse research sites). We will also examine sex differences in these effects. Ultimately, this project will inform policy and health campaigns regarding the role of the eCB system in adolescent developmentandprovidemechanisticinformationregardingthelong-termimpactofCANexposureinyouth.